The complement system plays a critical role in the antibody response to both thymus-dependent and thymus-independent antigens. Recently, the C3d fragment of complement C3 has been shown to have a profound adjuvant effect when coupled to a model T-dependent protein antigen and used to immunize mice. However, the utility of this approach for enhancing the humoral immune response to either T-independent or clinically relevant antigens has not been addressed. This project will test the hypothesis that C3d conjugated to the capsular polysaccharides of Streptococcus pneumoniae, clinically important T-independent type 2 antigens, has an adjuvant effect similar to that seen for T-dependent antigens. Preliminary data show that conjugation of C3d to pneumococcal serotype 14 capsular polysaccharide results in a significant enhancement of the murine antibody response to this polysaccharide. The ultimate goal of these studies is to gain an understanding of the immune response to C3d-polysaccharide conjugates with protein-polysaccharide conjugates in pneumococcal vaccines. The aims of this research are as follows. First to determine the effects of C3d conjugation on the humoral immune response to pneumococcal capsular polysaccharides as compared with the response to polysaccharides coupled to ovalbumin, a T-dependent protein carrier. The specific questions that will be addressed as part of this aim as follows: 1) Are there qualitative differences in the antibodies produced in response to immunization with the different types of polysaccharide conjugate? 2) What is the role of the complement system in the humoral immune response to the different conjugates? 3) What is the role of T lymphocytes in the antibody response to C3d-polysaccharide versus ovalbumin-polysaccharide conjugates? 4) Is the presence of a spleen necessary for mice to mount an effective humoral immune response to polysaccharide conjugates? 5) Within the spleen of immunized mice, how does the presence of C3d or ovalbumin coupled to pneumococcal polysaccharides influence the identity and distribution of the cells involved in the immune response? The second aim is to determine whether the adjuvant effect of C3d is a general phenomenon with respect to pneumococcal capsular polysaccharides. Conjugates of mouse C3d and capsular polysaccharide from serotypes 14, 6B and 23F S. pneumoniae will be prepared and evaluated in different strains of mice. These experiments should provide new insights into the immune response to conjugate vaccines and facilitate future efforts to improve vaccine efficacy.